To study the possible influence of mechanical ventilation on the accuracy of thoracic electrical bioimpedance (TEI) in the measurement of cardiac output, we determined cardiac output concurrently by TEI using Kubicek's equation and by thermodilution in 8 acutely ill patients who were mechanically ventilated (assist/control mode) but who had no underlying respiratory failure. Cardiac outputs were lower with TEI than with thermodilution (3.97 +/- 0.80 vs 4.83 +/- 1.16 l/min p = 0.004) and there was poor correlation between the values (r = 0.41). Although there is a need to develop non-invasive techniques to measure cardiac output, the present study indicates that TEI is not reliable in mechanically ventilated patients.